sql: don't create old when changing between hash sharded indexes

Fixes #45889.

Release note (sql change): This PR causes primary key changes to
not create a copy of the old primary key if the primary key change
is done from a hash sharded index into another hash sharded index.

pkg/sql/alter_table.go

@@ -563,7 +563,9 @@ func (n *alterTableNode) startExec(params runParams) error {
 			// does, but doesn't store anything. We only recreate the index if
 			// the table has a primary key (no DROP PRIMARY KEY statements have
 			// been executed), and the primary key is not the default rowid key.
-			if n.tableDesc.HasPrimaryKey() && !n.tableDesc.IsPrimaryIndexDefaultRowID() {
+			if n.tableDesc.HasPrimaryKey() &&
+				!n.tableDesc.IsPrimaryIndexDefaultRowID() &&
+				!(n.tableDesc.PrimaryIndex.IsSharded() && t.Sharded != nil) {
 				oldPrimaryIndexCopy := protoutil.Clone(&n.tableDesc.PrimaryIndex).(*sqlbase.IndexDescriptor)
 				// Clear the name of the index so that it gets generated by AllocateIDs.
 				oldPrimaryIndexCopy.Name = ""

pkg/sql/logictest/testdata/logic_test/alter_primary_key

@@ -961,3 +961,18 @@ t2  CREATE TABLE t2 (
     FAMILY fam_0_x (x),
     FAMILY fam_1_y (y)
 ) INTERLEAVE IN PARENT t (x)
+
+# Regression for #45889.
+statement ok
+DROP TABLE IF EXISTS t CASCADE;
+CREATE TABLE t (x INT PRIMARY KEY USING HASH WITH BUCKET_COUNT = 2);
+ALTER TABLE t ALTER PRIMARY KEY USING COLUMNS (x) USING HASH WITH BUCKET_COUNT=3
+
+query TT
+SHOW CREATE t
+----
+t  CREATE TABLE t (
+   x INT8 NOT NULL,
+   CONSTRAINT "primary" PRIMARY KEY (x ASC) USING HASH WITH BUCKET_COUNT = 3,
+   FAMILY "primary" (crdb_internal_x_shard_2, x, crdb_internal_x_shard_3)
+)